1. Field of the Technology
The disclosure relates to the field of wireless network routing, specifically routing protocols that perform intelligent cross-layer routing among equal cost multi-paths.
2. Description of the Prior Art
Over the past few years, a wide variety of routing protocols have been developed for mobile ad hoc networks (MANETs). These protocols can be generally classified into two categories: reactive (source driven) and proactive (table driven) protocols. Several previous Reactive Routing Protocols (RRPs) used in the past only create a route between a pair of source and destination nodes when the source node actually needs to send packets to the destination. While these protocols can avoid network wide topology information flooding, they are often subject to long latencies. In contrast, previous Proactive Routing Protocols (PRPs) in the past attempt at maintaining consistent and up-to-date routing information in each node by propagating updates throughout the network. Although a route to every other node is always available, such protocols introduce a significant bandwidth overhead due to the broadcast nature of exchanging routing information over wireless media. Still further proposals previously implemented are meant to reduce the overhead associated with routing information exchange by adding capabilities appropriate to radio interfaces while keeping the behavior on other interfaces untouched.
A cross-layer routing protocol utilizes link quality information available at the MAC/PHY layer to directly or indirectly make routing decisions. The media access control (MAC) data communication protocol sublayer, is a sublayer of the data link layer specified in the seven-layer OSI model (layer 2). Layer 1 is the physical layer, PHY. It provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multiple access network that incorporates a shared medium, e.g. Ethernet. The hardware that implements the MAC is referred to as a medium access controller. The MAC sub-layer acts as an interface between the logical link control (LLC) sublayer and the network's physical layer. The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast, multicast or broadcast communication service.
Among new cross-layer techniques, previous works directly incorporate link quality information into the calculation of the cost, while the still other works detect the undergoing link effects by injecting probing packets. However, both set of these previous approaches ignore the difference between the time scales of link quality variations and routing table discovery/update latencies. This problem is commonly referred to as the “Time Scale Difference” (TSD) problem. Routing updates typically occur over time scales in the order of seconds, while wireless link effects occur over time scales in the order of milliseconds. In the case of reactive routing protocols, the best link quality path identified at the time of discovery may no longer be that path at the time of packet forwarding. In the case of proactive routing, routing tables periodically converge after each node receives the topology updates. The update interval is usually set to several seconds to avoid oscillations. Thus, there is no way to guarantee the link is in its high quality state upon arrival of a packet at an intermediate node. In particular, PRPs are capable of performing Equal Cost Multi-Paths (ECMP) routing, which is typically used for load balancing among redundant paths in wired networks. In MANETs, however, the costs of all ECMP paths are highly unlikely to be equal considering time-varying link qualities.
A wide variety of techniques have emerged to improve routing efficiency in MANETs, focusing on two categories of works: techniques utilizing the link quality information, and those using the concept of hybrid routing.
A large body of work related to techniques utilizing the link quality information is routing metric oriented. These works mostly focus on developing new routing metrics that can represent link quality information available at PHY and MAC layers. Other works directly utilize the link quality information computed at MAC/PHY, while still other previous attempts indirectly probe the link quality in terms of packet loss rate. Both adaptive link-weight routing and routing in ad hoc networks with multiple-input multiple-output links take a step further by introducing bidirectional cross-layer approaches. Other works define and evaluate the performance of a triple-metric of rate, interference, and packet success rate using a cross-layer approach.
Zone Routing Protocol (ZRP) for ad hoc networks utilizes the concept of hybrid route discovery and selection. ZRP only uses a proactive approach to collect local topology information without generating a global routing table. The reactive component of ZRP performs global route discovery through the multicast based “bordercast” mechanism in order to propagate route queries throughout the network as oppose to relying on neighbor-broadcast flooding found in traditional reactive protocols.
What is needed is an improved routing efficiency by performing intelligent cross-layer routing among ECMPs. In the absence of ECMPs and given an individual hop selected in the shortest path route, there might exist alternatives with a higher number of hops but lower packet loss rate and delay. By identifying such alternatives, routing efficiency can be improved by performing local hop-based on-demand search.